Recent practice has been to converge a laser beam to a beam spot having a diameter of about 1 .mu.m and thereby to record and reproduce signals on and from an optical disc, respectively. Particularly, there has come into use an apparatus in which a recording material is evaporated onto an optical disc and a thermal change is caused in the recording material by applying a laser beam thereonto to thereby attain recording.
An example of a conventional optical recording and reproducing apparatus of the above-mentioned type will now be described with reference to the drawings.
FIG. 1 shows the construction of a conventional optical recording and reproducing apparatus. The laser beam from a semiconductor laser 1 is concentrated by a condenser lens 2 and then projected onto a convergent lens 5 through a total reflection mirror 4. The convergent lens 5 converges the incident beam to a tiny beam spot of about 1 .mu.m .phi. on an optical disc 8. The reflected beam from the optical disc 8 is passed again through the convergent lens 5 and the total reflection mirror 4 and sent to a beam splitter 3, from which it is received by a light detector 6. Numeral 7 designates a disc motor which rotates the optical disc 8. Numeral 9 designates a preamplifier for amplifying the output signal from the light detector 6 and applying it to a digital demodulator 11 through a signal processing circuit 10. Numeral 12 designates a semiconductor laser drive circuit which is responsive to the signal from a modulator 13 during a recording process to drive the semiconductor laser 1 by an electric current having a value sufficient enough to make its light output higher than the sensitivity of the recording material.
FIG. 2a shows the manner in which a signal is recorded along a guide track 14 in the form of a furrow or groove preliminarily formed on the optical disc 8 in the phase structure. FIG. 2b shows the waveform of an output signal of the preamplifier 9 generated by reproducing the thus recorded pits.
When the tiny beam spot is projected onto the recording material on the optical disc 8, the recording material is changed thermally and blackened as shown in FIG. 2a (the hatched portions). As shown by the output waveform of a reproduced signal in FIG. 2b, the blackened portions are increased in the reflectance (the reflectance increases in the upward direction (the positive direction) in the Figure). In other words, the signal is recorded at the portions of the recording material which are blackened to have an increased relectance. This recording is referred to as blackening recording. TeOx-type materials are used as suitable recording materials for the blackening recording. The TeOx-type materials allow recording therein, but erasing therefrom (restoration) is relatively difficult.
FIG. 3 explains recording materials suitable for the so-called whitening recording, and it shows the manner in which the pits are recorded along the guide track 14 like FIG. 2. In FIG. 3a, when the ting beam spot is projected onto the recording material on the disc, the material is changed thermally and whitened. As shown by the reproduced signal output waveform in FIG. 3b, the portions having the signal recorded therein and whitened are decreased in the reflectance (the reflectance also increases in the upward direction (the positive direction) in FIG. 3b as in the case of FIG. 2b). In other words, the signal is recorded in the portions which are whitened and decreased in the reflectance. This recording is referred to as whitening recording. Suitable materials for this system are TeGexOy-type materials which allow recording and erasing (restoration).
As described so far, the polarity of the reproduced output signal waveforms generated from the optical discs differs depending on the different recording materials. From the point of view of effectively utilizing optical discs, a recording and reproducing apparatus is not necessarily limited to its use of optical discs coated with one kind of recording material by the evaporation process. The optical disc of the blackening recording system incapable of erasing is suitable for the recording of information which should not be erased but should be retained permanently, and it is expected for the utilization as a data base. On the other hand, the optical disc of the whitening recording system capable of erasing is suitable for use as a working area of a memory in a computer.
Thus, there is a sufficient possibility for the same recording and reproducing apparatus to perform recording and reproducing on the optical discs employing two or more kinds of recording materials. The problem encountered in such a case is the fact that the polarity of a reproduced signal differs depending on the recording material as mentioned previously. The optical recording and reproducing apparatus constructed as shown in FIG. 1 is disadvantageous in that the apparatus cannot effect recording and reproducing on the optical discs employing recording materials which produce reproduced signals of different polarities.